Fix Seq2SeqTrainer generation path for decoder-only models

[shaealh]

Closes #44593

Summary

• use generation_input_ids/generation_attention_mask when provided for decoder-only models
• otherwise infer prompt from leading -100 labels and build left-padded prompt batch
• return completion tokens for decoder-only generation (strip prompt)
• keep encoder-decoder behavior unchanged

Tests

• PYTHONPATH=src python -m pytest tests/trainer/test_trainer_seq2seq.py -k Seq2SeqTrainerPredictionStepTester -q -rs

Related #26474, #33396
Follow-up to #32346

[Rocketknight1]

cc @SunMarc

[shaealh]

Still waiting for review and approval

[shaealh]

following up here. Still waiting for approval